The above-mentioned hand-held compact work apparatuses using a compact internal combustion engine, which may typically be a single-cylinder engine, include brush cutters used in mowing, chain saws intended for use in logging or trimming, and hedge trimmers used in hedge trimming, all of which are well-known. These brush cutters, chain saws, etc. use a cutting blade that moves at a high speed. For this reason, there have been developed work apparatuses equipped with a safety device intended to protect the worker using the work apparatus from the cutting blade when any sudden danger arises during the work.
Japanese Utility Model Laid-open Publication No. H02 (1990)-131822 (hereafter referred to as “Patent Document 1” as well) discloses a shoulder-strap type brush cutter, i.e. a hand-held brush cutter. On each of right and left handles of the brush cutter, a stop lever is provided. The stop lever is actuated when grasped by the worker. When at least one of the right and left stop levers is released, fuel supply to the engine is forcibly stopped by closing a fuel valve mechanically linked to the stop lever, or supply of power to an ignition plug of the engine is forcibly interrupted.
Japanese Patent Laid-open Publication No. H04-(1992)-158714 (hereafter referred to as “Patent Document 2” as well) discloses a shoulder-strap type brush cutter in which a stop lever or pressure sensor is provided on a handle of the brush cutter to actuate an electromagnetic brake or mechanical stopper and thereby forcibly stop the rotation of the cutting blade when the worker unlinks off the handle.
Japanese Patent Laid-open Publication No. H08 (1996)-187024 (hereafter referred to as “Patent Document 3” as well) discloses a shoulder-strap type brush cutter, i.e. a hand-held brush cutter, having a centrifugal clutch interposed between a drive source and a power transmission shaft that mechanically connects a cutting blade to the drive source. The centrifugal clutch has a clutch shoe that mechanically operates by a large relative displacement, if any, between the drive source and the power transmission shaft to automatically brake the centrifugal clutch.
Japanese Patent Laid-open Publication No. 2008-118960 (hereafter referred to as “Patent Document 4” as well) discloses a shoulder-strap type brush cutter having a cap attached at an end of the shoulder strap such that the cap can be brought into engagement with a switch body on the part of the brush cutter. In this shoulder-strap type brush cutter, if the cap disengages from the switch body upon a kickback of the brush cutter that may occur when the cutting blade hits a large stone or rock, power supply to an ignition plug is forcibly interrupted, and the engine stops.
Japanese Patent Laid-open Publication No. 2006-288296 (hereafter referred to as “Patent Document 5” as well) discloses a hand-held type electric brush cutter having a shock sensor. The shock sensor includes a piezoelectric element that abuts a circumferential part of a manipulation tube provided around a power transmission shaft connecting an electric motor as a drive source to a cutting blade. When the shock sensor detects a shock larger than a predetermined level, the motor is forcibly stopped. More specifically, a box surrounding the manipulation tube is provided near a handle fixed at a lengthwise central portion of the manipulation tube, and the shock sensor is affixed to the box in abutment with the manipulation tube. In this Patent Document 5, it is also proposed to provide, in the safety device including the shock sensor, a variable resistor for adjusting a current flowing through the shock sensor (piezoelectric element) to eliminate the variation in sensitivity from one piezoelectric element to another.